Cycloolefin copolymer or “COC” is a relatively new commercial polymer. Four commercial sources are available under the TOPAS®, Apel™, Zeonor™ and Zeonex®, and Arton™ product names, supplied by Topas Advanced Polymers, Mitsui Chemicals, Zeon Chemical and Japan Synthetic Rubber. Commercial COC is typically a random copolymer of ethylene and norbornene. Norbornene is synthesized via Diels-Alder reaction of ethylene and cylopentadiene. Polymerization of ethylene and norbornene using metallocene catalysts produces cyclic olefin copolymer. Large bulky cyclic rings randomly distributed in a polyethylene backbone prevent crystallization of the ethylene, creating an amorphous morphology.
COC grades are distinguished by glass transition temperature (Tg) and molecular weight. The glass transition temperature depends on the mole percent of norbornene. Typical commercial Tg range is between 68 and 170° C. COC has many key property attributes, including but not limited to exceptional moisture and aroma barrier, chemical resistance, transparency, purity, stiffness and strength. U.S. Pat. No. 6,255,396 to Ding et al. discloses polymer blends for making films including cycloolefin copolymers and linear low density polyethylene (LLDPE). Additionally, LLDPE blends with an ethylene-norbornene copolymer are disclosed in U.S. Pat. No. 6,111,019 of Arjunan et al. Oriented film of similar composition is seen in U.S. Pat. No. 5,583,192 of Bennett et al. Cycloolefin copolymer layers have been used in multilayer articles, such as a flavor retaining multilayer plastic container disclosed by Itoh et al. in U.S. Pat. No. 6,042,906. See also Lamonte, R. “Stiffer, Thinner Packaging Films with Improved Sealing Using Cyclic Olefin Copolymers”, 10th Worldwide Flexible Packaging Conference, Amsterdam (November 2000) as well as “Optimization of Wall Thickness Distribution of Pharmaceutical Press-Through Blisters”, Wolf, J. et al., Institute for Plastics Processing (IKV), Aachen, Germany, ANTEC, 1999.
Thermoformable composite films which include a cycloolefin copolymer are seen in U.S. Pat. No. 6,329,047 to Beer et al. as well as U.S. Pat. No. 6,641,925 also to Beer et al. The composite film of these patents includes a cycloolefin copolymer layer and a polyvinylidene chloride (PVDC) layer. The thermoformable composite film is proposed for blister packs. See also Gebranchsmusterschrift DE 20116341 U1.
The following patents all to Bravet et al. disclose a transparent window formed of cycloolefin copolymer and a scratch resistant layer: U.S. Pat. Nos. 7,101,611; 6,998,169; and 6,811,857.
Despite advances in the art, there exists a need for thermoformed articles which have high stiffness and strength, superior optical and barrier properties, and processability as characterized by low gauge variation and resistance to shrinkage. Processing and product issues become particularly acute when trying to form so called “deep-draw parts” where ductility of the sheet is a salient feature.